[arp] Try to avoid discarding ARP cache entries

[arp] Try to avoid discarding ARP cache entries

Discarding the active ARP cache entry in the middle of a download will
substantially disrupt the TCP stream.  Try to minimise any such
disruption by treating ARP cache entries as expensive, and discarding
them only when nothing else is available to discard.

Signed-off-by: Michael Brown <mcb30@ipxe.org>